1. Field of the Invention
The present invention relates to a liquid jet recording head which forms liquid droplets by emitting liquid, for the purpose of image recording.
2. Description of the Prior Art
Non-impact recording methods are attracting attention in that noise at recording is negligibly low. Among such methods, a liquid jet recording method (ink jet recording method) is particularly promising in that it is capable of high-speed recording on plain paper without a particular fixing step. For this reason, there have been proposed various working principles and corresponding devices, some of which are already in commercial application while others are still in the course of development.
Among such ink jet recording methods, a method disclosed in the Japanese Patent Laid-open Sho No. 54-51837 and the German Patent Laid-open (DOLS) No. 2843064 is different from others in that the force for forming flying liquid droplets is obtained by thermal energy applied to liquid.
According to the description of the above-mentioned patent specifications, liquid receiving the thermal energy causes a state change involving a rapid volume increase, including formation of bubbles, thereby emitting liquid droplets from an orifice of a recording head tip, and said droplets fly and attach to a recording member to form a recording thereon.
Particularly the ink jet recording method disclosed in the DOLS No. 2843064 is not only effectively adaptable to a so-called drop on demand recording method but also can be realized easily into a full-line multi-orifice recording head of a high density, thus enabling providing an image with high resolution power and high quality with a high speed.
FIG. 1 (A) is a schematic perspective view of a conventional liquid jet recording head, and FIG. 1 (B) is a cross-sectional view thereof along a line A--A in FIG. 1 (A). A head 101 comprises a substrate 103 equipped thereon with electrothermal converting members 102 for generating the liquid emitting energy, wall members 104 joined thereon and a plate member 105 joined thereon to form liquid flow paths 106 and a liquid chamber 107, into which a recording liquid is supplied through a liquid supply entrance 108 formed in said plate member 105. Thermal energy caused by said electrothermal converting member 102 on the substrate 103 through electric power supply to said converting member is transferred to the recording liquid occupying the liquid flow path 106 to generate bubbles in the recording liquid, and the resulting rapid volume increase causes the recording liquid to be emitted from a liquid emitting port 109 at the end of said liquid flow path 106, thus generating flying liquid droplets. In such process, the liquid emitting force is significantly affected by the balance of flow resistances in the flow path in front of and behind the electrothermal converter 102 functioning as a thermal energy generator in the liquid flow path 106. More specifically, in order to effectively utilize, as the liquid emitting energy, the pressure increase in the liquid flow path 106 caused by the volume change of the bubbles on said thermal energy generator, and to minimize the loss of said pressure increase resulting from pressure dissipation into the liquid chamber 107, it is necessary to design the liquid flow path between the thermal energy generator and the liquid chamber 107 longer than the flow path between said thermal energy generator and the liquid emitting port 109, thus increasing the flow resistance in the flow path leading to the liquid chamber 107 and increasing the flow energy of the recording liquid toward the liquid emitting port.
However, after the emission of recording liquid droplets, the recording liquid is replenished in the liquid flow path 106 by being pulled thereinto from the liquid chamber 107, by the surface tension of the meniscus of the recording liquid maintained at the liquid emitting port 109. Consequently, since said surface tension of the meniscus can only exert a determined amount of energy, the aforementioned increase of the flow resistance in the liquid flow path 106 inevitably reduces the flow rate of the recording liquid per unit time in the flow path 106. This fact increases the time required for supply of recording liquid into the flow path 106, and deteriorates the frequency response for printing by emission of liquid droplets. The frequency response, or high-speed recording performance, of the liquid jet recording head as shown in FIG. 1 is excellent in comparison to that of other liquid jet recording heads, thermal printers, thermal transfer printers, wire dot printers etc., but is inferior to that of still faster printers such as laser beam printers and electrophotographic copiers. Consequently, it is desired to develop a technology allowing high-speed recording comparable to the performance of such printers.
An important factor in the commercial use of the liquid jet recording head is the dependability and stability of the recording operation after the recording head is left unused for a prolonger period. The above-explained conventional liquid jet recording head, utilizing long and narrow liquid flow paths containing only a small amount of recording liquid in the vicinity of the thermal energy generator, may become unable to emit liquid droplets after a prolonged rest, since the solvent in the recording liquid may evaporate from the liquid emitting port to elevate the viscosity of said recording liquid and the flow resistance thereof in the flow paths. A similar phenomenon may be encountered at a low temperature when the recording liquid becomes more viscous. For this reason it has been proposed to provide the supply system of the recording liquid with a pump to feed the recording liquid of even a high viscosity, or to start the recording operation after various preparatory liquid emissions. However these measures have induced various drawbacks such as increased cost and lowered reliability due to a more complicated mechanism, increased running cost due to the waste of recording liquid, and inability to immediately print due to certain preparatory operations.